1. Field of the Invention
The present invention relates to a system for protecting an electrical power system, and more particularly, to an electrical power system protecting method and device in which, in the event of a failure in a part of electrical lines of an electrical power network or system, a suitable measure is taken for ensuring safe operation of other sound lines.
2. Description of the Related Art
In general, electrical power network is constructed to include a plurality of power transmission lines or a plurality of power distributing lines. In the event of a failure such as short-circuit fault in one of the power lines of the network, the line in which the failure has occurred is disconnected from sound power lines which are on the "upstream" side of the failed line by tripping the circuit breaker of the receiving end of the failed line.
In the case of a power distribution system, high voltage such as 77 KV is transformed into a lower distribution voltage, e.g., 6.6 KV, through one or more transformers which are provided in a distributing substation, and the power of the thus reduced voltage is distributed from a substation BUS to a plurality of feeders branching off the substation BUS. The power is then supplied to consumers through the feeders. In this type of distribution system, when a failure such as short-circuit fault has occurred in the load side of one of the feeders, the circuit breaker provided on the branching end of the feeder adjacent to the substation BUS (referred to as "feeder circuit breaker") is made to trip so as to disconnect this feeder, while ensuring supply of electrical power to other feeders which are sound, thus protecting the power distribution system. A protecting method called "harmonic protection" also has been adopted in which circuit breakers in the failed feeder which is most downstream and which is closest to the point of failure is selectively and preferentially opened to enable supply of power to wide area as possible. For instance, the timing of tripping of the feeder circuit breaker is delayed a predetermined time, e.g., 0.2 second, after tripping of the circuit breaker of the receiving end of a consumer in which the fault has occurred.
In the conventional protecting system as described, when the point at which the fault has occurred is close to the feeder circuit breaker, a large short-circuit current may flow through this feeder during the period of the delay mentioned above. As a consequence, the voltage of the power supplied to other feeders which are sound and which are connected to the same main transformer as the failed feeder may drop. Such a drop of the voltage has not been recognized as a critical problem. In recent years, however, the influence of such voltage drop is often not negligible because the short-circuit current is very large in current power network having a large capacity. In particular, in the modern information-oriented society in which abundant electronic information devices such as computers are used, a large voltage drop causes a serious situation such as erasure of the information stored in terminal devices of information systems, on-line control systems and computers, even when the period of such voltage drop is very short.
On the other hand, an increase in the level of short-circuit current attributable to increase in the capacity of electrical power network requires that mechanical durability of equipments in the network, as well as the capacities of the circuit breakers in the network, to be increased. In recent years, therefore, a current limiting device has been proposed which suppresses increase in the level of short-circuit current thereby reducing breaking capacities required for circuit breakers, as in, for example, in Report SPD-87-43 of The Institute of Electrical Engineers of Japan, Oct. 27, 1987. In this current limiting device, a current -limit fuse and a current limiting element are connected in parallel with a switching element which has a predetermined breaking capacity. In the event of a short-circuit fault, the switch element is opened first so that short-circuit current flows first through the current limiting fuse. After a while, the current limiting fuse is fused so that the short-circuit current is made to flow through the current limiting element, whereby the level of the short-circuit current is limited. This current limiting device suppresses the level of the short-circuit current so as to diminish the reduction of voltage in the sound portion of the electrical power network in the event of a fault of the kind described.
The art disclosed in the Report mentioned above, however, suffers from the following problem. Namely, the above-mentioned current limiting device, when used as a feeder circuit breaker, is not designed for performing the aforesaid harmonic protection function, i.e., selective and preferential tripping of circuit breaker which is downstream from the feeder and which is close to the point where the fault has occurred. Therefore, when the short-circuit current is limited excessively, the momentary element of an over-current relay which is to trip the circuit breaker closest to the point where the fault has occurred so not operate so that the time required for separation of the fault point is prolonged undesirably.